The Wilwood engineer is correct with regard to stopping.
The Siemens engineer said there were two parts; getting the wheels to stop is one, and what was the second?
My concern is that you now have a two-wheel hydraulic brake system with minimal fluid volume. The fluid only needs to hit 270 C/ 518 F to boil, and you lose the ability to stop. Holding a set speed on a downhill run for an extended period will heat brakes more than a highspeed panic stop. This is because, in a panic stop, the brakes have an opportunity to lose heat after you are stopped. With continued braking, the heat keeps building and can easily overwhelm the capacity (mass) of the brake disc to lose that heat. The 600-foot figure, I assume, is the elevation change. I was unable to find where you mention the distance over which you descend the 600 feet in any of your posts. Could you please let us know? Is it one continuous run, or are there stops along the way? What is the speed limit or desired maximum speed for the descent?
The engineers you spoke with took into account the speed and weight of the vehicle. I would suggest you consider a worst-case scenario. The number of occupants x the average weight of an adult male in your region + the estimated weight of the finished vehicle = Maximum Total Weight to be used when calculating the required mass for the brake discs.
In a modern car, you can rub the tires against the curb to scrub off speed, an option you don't have with wooden wheels.
